You can use the AnonymousId property of the AnonymousIdentificationEventArgs object supplied to the AnonymousIdentification_Creating event to set the anonymous identifier to a custom value. If you do not specify a value for the AnonymousId property during the AnonymousIdentification_Creating event, a Guid is used.

The AnonymousIdentification_Creating event is only raised when anonymous identification is Enabled by setting the anonymousIdentification configuration element to true.